1. Field of the Invention
This invention relates to a modular molding process for forming a composite piston for a hypodermic syringe wherein thermoplastic rubber and plastic parts of the piston are integrally formed in the same mold in a multi-phase, or modular, injection molding process, and wherein the composite piston is thereafter ready to be seated on the piston end of a conventionally made syringe shaft.
2. Description of the Related Art
Hypodermic syringes are comprised of a barrel, which receives a fluid to be injected, and a plunger, which transmits force via a piston rod to a piston for either drawing in or dispensing the fluid. The plunger is conventionally comprised of a rigid plastic thumb press, a flexible rubber piston for sealingly engaging the walls of the barrel, and a shaft for transmission of force between the thumb press and piston. Conventionally, plungers are constructed in three separate steps: a step of manufacturing a rigid plunger shaft, a step of manufacturing an elastic rubber piston, and a step of assembling the rubber piston onto the rigid plunger shaft using a variety of techniques including mechanical interlock, adhesive, ultrasonic welding, heat fusion or the like.
The pistons are conventionally comprised wholly of a resilient, thermoplastically processable rubber mixture, consisting predominantly of a block polymer, and can contain a quantity of a polyolefin. Such a composition is necessary to provide good resilience, while presenting minimal relaxation phenomenon in the course of use, thus insuring a fluid-tight seal between the piston and the inner surface of the syringe barrel. Further, such a composition is resistant to the most common medical agents. Unfortunately, such rubber mixtures are much more expensive, on a weight basis, than the materials used to construct the other portions of the syringe.
The rubber pistons are typically mass produced in at least four separate steps: a step of heat-impression stamping a rubber sheet around core pins, tumbling the rubber pistons to debur them, and to eliminate any flash created during the stamping process, washing the pistons to eliminate the dust and debris created in the tumbling process, and lubricating the finished piston.
In terms of time and efficiency, this multi-step piston-manufacturing process is costly and requires quality control. Further, when the rubber sheet is heat-impression stamped around core pins, latent variations in rubber density are created in the piston. These density variations may be attributed either to unavoidable folding of the rubber sheet around the core pins or to variations of density in the rubber sheet itself. These density variations (1) allow the plunger rods to pull out of the pistons thereby resulting in a 5-7% failure rate in the syringe and (2) interfere with the fluid-tightness of the seal between the piston and the syringe barrel, allowing failure in operation such as in an emergency room.
There is also a problem of balancing the softness properties of the rubber-if the rubber is too hard, the seal forming properties are impaired, and if the rubber is too soft, the piston will come unseated from the plunger shaft during drawing in of fluids. In a hospital emergency room, speed and reliability are a precious commodity. Accordingly, it is absolutely critical that hypodermic needles meet a high standard of reliability and performance, and any feature that will contribute to reliability of the syringe would be advantageous.
If these reliability considerations could be met, then it would be desirable to have a process for producing a syringe plunger piston with significantly lowered production costs.